WASHINGTON — Senate Majority Leader Harry Reid, D-Nev., in a showdown with the White House over executive branch nominations, Friday refused to formally adjourn the chamber for a planned two-week Thanksgiving break in order to thwart President Bush’s ability to make recess appointments.

Rather than taking a full break, Reid employed a rarely used parliamentary tactic of scheduling “pro forma” sessions twice a week until early December, when Congress returns for a busy three weeks of work. Under that plan, a few senators, perhaps just one Democrat and one Republican, will briefly open the chamber for debate on any topic during the next two weeks.

This move blocks Bush’s ability to make so-called recess appointments, which would allow the appointees to serve out the remainder of Bush’s term.

Reid accused Bush of moving slowly on Democratic nominees for a host of bipartisan oversight agencies such as the Federal Communications Commission. Reid said he had made good on a summertime promise to move several key Bush nominees, including new Attorney General Michael Mukasey, but Bush has not officially nominated some Democratic selections for the bipartisan commissions.

“I am committed to making that progress if the President will meet me half way,” Reid said in a statement inserted in the Congressional Record. “But that progress can’t be made if the President seeks controversial recess appointments and fails to make Democratic appointments to important commissions.”

An administration official said Democratic nominees for the Federal Communications Commission, the Federal Election Commission and the National Labor Relations Board were close to being formally sent to the Senate.

The White House accused Democrats of holding up more than 200 executive and judicial branch nominations, including some for the circuit courts and the Federal Reserve. “If they are going to come in every three days, they might as well hold hearings on these nominees and make progress on filling these important positions,” said spokeswoman Emily Lawrimore.

Agencies such as the FCC are bipartisan. By tradition, congressional Republicans and Democrats recommend names to the White House for nomination to the Senate. The president usually selects the head of the commissions.

Under law, a president can use a recess appointment if the Senate is adjourned more than three days without reconvening on the fourth day. The interim appointments last through the current and next sessions of Congress.

Congressional rules allow for the Senate to be adjourned for three full days without being considered on a recess. Bush cannot use the interim appointments as long as the chamber is opened every fourth day. Reid set a schedule of pro forma sessions on Tuesday and Friday of next week, and then on Nov. 27 and Nov. 29 of the following week.

The Senate returns Dec. 3 for full legislative sessions and is expected to adjourn a few days before Christmas until mid-January. But Reid is threatening to hold pro forma sessions throughout the holiday season to block recess appointments if necessary.